Method for preparing nickel hypophosphite

ABSTRACT

Nickel hypophosphite is prepared by ion exchange on a carboxylic between nickel sulphate and hypophosphorous acid. The product thus obtained, both in the form of an aqueous solution and of a crystalline salt, is intended in particular for the preparation of chemical nickel plating baths.

TECHNICAL FIELD OF THE INVENTION

The present invention is that of a process for the manufacture of nickelhypophosphite.

PRIOR ART

Nickel hypophosphite is an inorganic salt known since at least 1828(Rose, Ann. Physik., [12-87-1828). It crystallizes from its aqueoussolutions in the hexahydrate form Ni(H₂ PO₂)₂.6H₂ O, It is registered inthe Registry file of Chemical Abstracts as RN 13477-97-9. Itsconstituent components, the hypophosphorous anion on the one hand andthe nickel cation on the other hand, are the active species of thewell-known chemical nickel plating process which makes use of theautocatalytic oxidation/reduction reaction.

NiSO₄ +2NaH₂ PO₂ +2H₂ O→Ni+2NaH₂ PO₃ +H₂ SO₄ +H₂ ↑ and the baths ofwhich are manufactured from sodium hypophosphite and a soluble nickelsalt, generally the sulphate. It might have been thought advantageous touse nickel hypophosphite for this end, with the obvious advantage ofnot, for no purpose, charging the baths with sodium and sulphate ionswhich cause difficulties due to the crystallization of sodium sulphate.

The Nouveau Traite de Chimie Minerale [New Treatise on InorganicChemistry] by Paul Pascal cites two processes for obtaining nickelhypophosphite, one by double decomposition between nickel sulphate andbarium hypophosphite and the other by dissolving nickel hydroxide inhypophosphorous acid. To convert the double decomposition process to anindustrial scale, it would be advisable to use calcium hypophosphite asa realistic source of hypophosphite; this results in a nickelhypophosphite which is unsuitable for chemical nickel plating because itcontains an excessively large amount of magnesium, the major impuritycontributed by the calcium hypophosphite itself, and of calcium, whichis not removed from the reaction mixture due to the not insignificantsolubility of the calcium sulphate formed. On the other hand, it is thestarting material contributing the nickel to the dissolution process,metallurgic nickel hydroxide or nickel carbonate hydroxide, which is thesource of contaminants by contribution of heavy metals or magnesium. Dueto the lack of an economically acceptable production process, the use ofnickel hypophosphite in chemical nickel plating has remained highlyrestricted.

DISCLOSURE OF THE INVENTION

The process according to the invention overcomes these disadvantages. Itconsists in retaining the nickel of an aqueous solution of a solublenickel salt on an ion exchange resin and in eluting it with an aqueoushypophosphorous acid solution. It is thus possible to obtainhypophosphorous solutions assaying 30-35 g/l of nickel, which can beused directly to prepare or regenerate chemical nickel plating baths orfrom which nickel hypophosphite hexahydrate can be crystallized.

The resin to be used for the invention is a carboxylic resin (weak acid)in the sodium form. This type of resin can be regenerated byconcentrated solutions of strong acids. It is thus possible to drawtherefrom regeneration solutions which are concentrated in exchangedions. After regeneration, the resin is in an acidic form which isunfavourable for adsorbing nickel ions: it is brought back to thecarboxylate form, and preferably sodium carboxylate form, by treatmentwith a strong base, in this case sodium hydroxide. This is a highlyexothermic reaction (approximately 80 kcal/mol). The chemical nature ofthe resin is of little importance but its composition must be such thatit satisfactorily withstands the osmotic shocks created by thedifference in the concentrations of the various reactants which succeedone another in the column during the operation, the thermal shocks dueto the exothermicity of the neutralization of the acid resin, and thephysical effects resulting from the fact that, in very concentratedsolutions, the resin begins to float.

All water-soluble nickel salts are suitable for the invention. Nickelsulphate is preferred because it is easy to remove the sulphate ion fromthe effluent via a salt which precipitates this anion, for example acalcium salt. The pH of the nickel salt solution must be as close aspossible to 6 in order for the resin not to experience a loss incapacity because of an excess of H⁺ ions. The addition of acid will thusbe avoided, which is sometimes carried out in order to prevent theprecipitation of nickel hydroxide and to accelerate the dissolution ofthe salt.

The source of hypophosphite ions of the invention is hypophosphorousacid. Sodium hypophosphite is not effective in extracting the retainednickel ions, at least to an acceptable concentration level of nickelions in the effluent.

The operating temperature is partly set by the concentrations of thesolutions (nickel salt, hypophosphorous acid, sodium hydroxide) and bythe rate of their introduction into the column: it is necessary toprevent the resin being detrimentally affected by an excessive rise intemperature due to the interaction of excessively high concentrationsand throughputs. Moreover, with an excessively high throughput, ionexchange becomes partial. It is up to the person skilled in the art toachieve a correct balance under the conditions which are available tohim. Attempts should be made neither to exceed 80° C. nor to fall below25° C. and in any event an excessively low temperature is avoided duringthe phase of elution of the nickel with hypophosphorous acid, in orderto prevent the hypophosphite from precipitating in the column.

The process is quantitative with respect to nickel. It provides aqueousnickel hypophosphite solutions at concentrations varying from 1 to 35g/l of nickel. These solutions can be used directly by nickel platers inpreparing their chemical nickel plating baths. It is possible to addcomplexing agents in order to prevent highly concentrated solutions(more than 30 g/l of NiSO₄) from crystallizing when cold. It is alsopossible to crystallize these solutions in order to remove nickelhypophosphite hexahydrate therefrom, for example by evaporation undervacuum at moderate temperature (<50° C.), in order to avoid oxidation,and then crystallization by controlled cooling.

The nickel hypophosphite resulting from the process according to theinvention exhibits the appreciable advantange, for nickel plating, oflimiting to a very substantial degree the impurities introduced into thebaths by the reactants contributing the hypophosphite and nickel ions.This may be convincingly shown by considering the comparative contentsof the conventional reactants (sodium hypophosphite and nickel sulphate,on the one hand, and nickel hypophosphite according to the invention, onthe other hand):

    ______________________________________                                                 Sodium         Nickel  Nickel                                          Impurity hypophosphite sulphate hypophosphite                               ______________________________________                                        Ca         20-40   ppm              11.5 ppm                                    Fe <3 ppm 40 ppm <1 ppm                                                       Pb <1 ppm  1 ppm <1 ppm                                                     ______________________________________                                    

EXAMPLE

The reactants

0.856 mol/l nickel sulphate solution: 225 g (0.856 mol) of NiSO₄.6H₂ Oare dissolved in one liter of demineralized water.

2.5 mol/l hypophosphorous acid solution: 6 liters of 50% H₃ PO₂[d=1.21], i.e. 9.2 mol, are diluted with 14 liters of demineralizedwater.

Standard 30% sodium hydroxide solution.

Macroporous Amberlite IRC 76 ion exchange resin.

The procedure

1 liter of resin is charged, in a column of at least 1 l 25, with 770 mlof 0.856 mol/l nickel sulphate solution, i.e. 38.7 g of Ni²⁺. Rinsing iscarried out with demineralized water (approximately 1.5 liters) untilsulphate is no longer present in the rinse water. The nickel isdisplaced with 770 ml of 2.5 mol/l hypophosphorous acid (during thisoperation, the liter of resin contracts to 770 ml) and rinsing is thencarried out with demineralized water. 1060 ml of eluate and of rinsewater, which contain 38.7 g of Ni²⁺, are recovered in the form of asolution containing 0.622 mol/l of Ni²⁺ and 1.866 mol/l of H₂ PO₂ ⁻(hypophosphite/nickel molar excess of 1.5).

The resin is regenerated by charging with 230 ml of 30%sodium hydroxideand then rinsing with demineralized water to neutral pH.

A resin is thus obtained which is ready to be employed for receiving afresh charge of nickel sulphate.

We claim:
 1. A process for the preparation of nickel hypophosphite,characterized in that the preparation is carried out by ion exchange onan ion exchange column between a water-soluble nickel salt andhypophosphorous acid.
 2. A process according to claim 1, characterizedin that the ion exchange resin is a carboxylic resin in the sodium form.3. A process according to claims 1, characterized in that the nickelsalt is nickel sulphate.
 4. A process according to claim 2,characterized in that the nickel salt is nickel sulphate.